The use of microstructures as sensors, motors, gears, levers and movable joints in integrated circuits is becoming increasingly common. In the automotive industry, microstructure sensors capable of sensing mechanical variables such as acceleration are being used widely in the construction of anti-lock brake systems. The silicon diaphragm pressure gauge, a microstructure useful in monitoring fluid flow, is presently manufactured in large quantities. Microchemical sensors are expected to have wide spread application in demanding environments where small amounts of a chemical must be sensed and where conventional sensing devices are too large.
Along with the increasing demand for microstructures, there is also a demand for ever smaller microstructures. Although at present microstructures may be characterized by dimensions of upwards of 1000 .mu.m (1000 microns) and as small as 1 .mu.m or smaller, as industry moves toward ever smaller geometries, it is expected that the size of microstructures will continue to shrink.
With the development of microstructures and ever more intricate micromachines, new engineering problems arise that are unique to the microsizes involved. One such common and costly problem in the micromachining industry is stiction, which can occur during the release of free-standing microstructures by removing sacrificial layers used to support the free-standing microstructures when they are being constructed. Typically, sacrificial materials such as silicon dioxide are removed in a so called `wet release method` by use of an aqueous hydrogen fluoride solution. Stiction occurs when liquid, such as aqueous hydrogen fluoride or rinse solutions, comes into contact with microstructures causing the microstructures to stick to one another or to the substrate. This can occur either during or after the release process. Moreover, this phenomena is not limited to semiconductor substrates but may occur on other substrates as well.
Solutions to the problem of stiction include the use of micromechanical temporary supports, sublimation of the final liquid by plasma ashing, removing the final liquid through the supercritical state, the use of low surface tension liquids and photon assisted release methods. An example of the use of micromechanical temporary supports may be found in U.S. Pat. No. 5,258,097 to Mastrangelo wherein temporary posts or columns are erected to support the microstructure. Unfortunately, techniques such as this add additional costs to the fabrication of chips; as the desired structures become increasingly intricate, the design of dry release methods will become more complex and expensive. Moreover, as with all of these release techniques, stiction can recur should a subsequent process step introduce moisture into the system once the structure has been released.
Currently, the process of unsticking stuck structures is time consuming and laborious. Stuck structures are freed by physically manipulating the structures with a probe. Because of the size of the structures, this process must be carried out under a microscope. Accordingly, there is a need in the art for a novel method of freeing stuck microstructures which avoids the necessity of unsticking the individual structures one-at-a-time in a painstaking process.
The present invention offers a method for eliminating stiction by cooling the microstructure and subjecting it to a force. One such method involves the use of cryogenic aerosols. Cryogenic aerosol technology has been developed as a cleaning means for substrates. U.S. Pat. No. 4,747,421 to Hayashi describes an apparatus for cleaning substrates using carbon dioxide aerosol particles. U.S. Pat. No. 5,294,261 to McDermott et al., the contents of which are incorporated herein by reference, discloses a method for cleaning microelectronic surfaces using an aerosol of at least substantially solid argon or nitrogen particles. Copending US application, titled "Treating Substrates by Producing and Controlling a Cryogenic Aerosol" of Patrin et al., filed contemporaneously with the present application, and assigned to the same assignee, the contents of which are incorporated herein by reference, discloses an improved method for forming a cryogenic aerosol at low chamber pressure. U.S. Pat. No. 5,378,312 to Gifford et al., the contents of which are incorporated herein by reference, discloses a method of fabricating a semiconductor structure which includes the use of a cryogenic jet stream for the removal of films from the surface of the semiconductor. The present invention, in one embodiment, applies the technology of cryogenic aerosols to the problem of stiction with surprisingly good results.